1. Field of the Invention
Embodiments of the present invention relate generally to elements which sense or measure strain associated with the deflection of a substrate on the order of microns. Such substrates may include a beam secured at two ends or a cantilever secured at one end. More particularly, embodiments of the present invention relate to a strain-sensing or strain-measuring structures formed from semiconductor materials which exhibit a piezoelectric effect when subject to stress.
2. Description of Related Art
Arrangements incorporating electromechanical transducers and strain sensors are known. See, for example, U.S. Pat. No. 4,522,072, hereby incorporated by reference in its entirety, which describes a cantilever beam accelerometer and several separate silicon-based strain sensing elements connected by wheatstone bridges. The sensing elements may be field effect transistors (FET) which are generally known to be voltage controlled devices in which the current conduction between a source region and a drain region through a channel region is controlled or modulated by means of a control voltage applied to a gate terminal.
Other silicon based strain sensors are also known which incorporate a piezoelectric material as an integral part of a non-piezoelectric FET. The principle of operation results from a direct interaction between the piezoelectric material and the channel region of the FET. These types of strain sensors are fashioned from at least two different materials: a first material for the FET and a second material for the piezoelectric region incorporated into the FET.
Micromachined gallium-arsenide (GaAs) systems have been used to produce micromechanical structures. See, for example, AlGaAs/GaAs Micromachining for Monolithic Integration of Micromechanical Structures with Laser Diodes, by Y. Uenishi, H. Tanaka and H. Ukita, IEICE Trans. Electron., E78-C, No. 2, 139-145 (1995); Submicron, Movable Gallium Arsenide Mechanical Structures and Actuators, by Z. Lisa Zhang, G. A. Porkolab and N. C. MacDonald IEEE MEMS, 72-77 (1992); and Bulk and Surface Micromachining of GaAs Structures, by Klas Hjort, Jan-.ANG.ke Schweitz and Bertll Huk., IEEE MEMS, 73-76 (1990), also hereby incorporated by reference. GaAs semiconductor materials exhibit piezoelectric properties, and these properties are known to affect FET parameters. See, Piezoelectric Effects in GaAs FET's and Their Role in Orientation-Dependent Device Characteristics, by P. M. Asbeck, C. P. Lee, and M. F. Chang, IEEE Trans. Electron Devices, ED-31, 1377-1380, (1984); Role of the piezoelectric effect in device uniformity of GaAs integrated circuits, by M. F. Chang, C. P. Lee, P. M. Asbeck, R. P. Vahrenkamp and C. G. Kirkpatrick, Appl. Phys. Lett., 45 (3), 279-281, (1984); and Improvement in GaAs MESFET Performance due to Piezoelectric Effect by T. Onodera, T. Ohnishi, N. Yokoyama and N. Nishi, IEEE Trans. Electron Devices, ED-32, 2314-2318, (1985) hereby incorporated by reference in their entirety. However, no strain measurement devices based on a GaAs FET are known to exist.
When the size scale of systems used to detect mechanical deflections becomes small, i.e. on the order of microns, the detection of the mechanical deflection of a substrate, such as a cantilever or a beam, presents a challenge for known displacement-type sensors which generally operate separately from the substrate being deflected to detect and measure the degree of physical displacement of the substrate. Currently most systems to measure deflection of a substrate rely on optical readouts of deflection which presents disadvantages if the sample being scanned is photosensitive. Alternative deflection sensing mechanisms have been developed including external readouts via capacitance, see Microlever with combined integrated sensor/actuator functions for scanning force microscopy, by J. Brugger, N. Blanc, Ph. Renaud and N. F. de Rooij, Sensors and Actuators A, 43, 339-345 (1994) and integrated internal readouts via piezoresistive cantilevers, (see Atomic resolution with an atomic force microscope using piezoelectric deflection, by M. Tortonese, R. C. Barrett and C. F. Quate, Appl. Phys. Lett. 62, 834 (1993)), and piezoelectric bimorph cantilevers, (see, Scanning force microscope using a piezoelectric microcantilever, by T. Itoh and T. Suga, J. Vac. Sei. Technol. B 12, 1581 (1994); Force measurement with a piezoelectric cantilever in a scanning force microscope, by J. Tansock and C. C. Williams, Ultramicroscopy 42-44, 1464 (1992); and Scanning Force Microscope Using Piezoelectric Excitation and Deflection, by T. Itoh, T. Ohashi and T. Suga, IEICE Trans. Electron, E78-C, No. 2, 146-151 (1995) which are used to detect surface forces between a nanometer scale tip and a sample surface based upon a measured piezoelectric effect resulting from the movement of the cantilever.
The strain produced by a force or by a displacement generally increases as the size of the mechanical structure is reduced. Thus, a strain sensor presents a desirable altemative to deflection sensors for detecting small forces and displacements in micro-mechanical systems. Scanned probe microscope (SPM) cantilevers provide one important example of a small system where a strain sensor could be used. Additionally, the strain produced as a result of movement of a microcantilever is greatest at the fixed base portion of the cantilever. Measurement of the strain at that portion of the cantilever, therefore, should be optimized to provide increased sensitivity and accuracy. Consequently, a strain-sensing cantilever structure is needed which overcomes the disadvantages of the prior art deflection sensors and which provides a sensing or measuring mechanism which overcomes the disadvantages and improves on the shortcomings of the simple bimorph piezoelectric cantilevers.